This application claims the priority of German application 100 38 724.1, filed Aug. 9, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a particular method and a particular system for catalytic aftertreatment of internal-combustion engine exhaust gas.
Reducing pollutants in the exhaust gas of a motor vehicle internal-combustion engine during an engine cold start and an engine warm-up is most promising for reducing pollutants when aftertreating the exhaust gas. Among other methods known for this purpose is a method in which, during a cold start, a combustible air/fuel mixture is generated in the exhaust gas train. The heating value of the exhaust gas is utilized to provide for rapid heating of the pollutant-converting catalysts to their starting or usage temperature. This takes place because the combustible air/fuel mixture is ignited again at an appropriate point in direct proximity to the catalyst.
A method for afterburning exhaust gas to rapidly heat a catalytic system is suggested in International Patent Document WO 93/07365. The corresponding system has two catalysts. A combustion chamber which has an ignition aid is arranged between the two catalysts. After cold starting the engine, the engine is operated in a rich manner, secondary air is admixed in front of the catalyst system, and the combustible exhaust gas/air mixture is ignited in the combustion chamber and burnt as completely as possible. The heat of the reaction is used for rapidly heating the catalyst. During the heating phase, exhaust gas in the combustion chamber is significantly warmer than the inflowing exhaust gas.
The principal disadvantage of this system is that the system may cool again after the heating phase, and the catalyst temperature may fall below the light-off temperature until the afterflowing exhaust gas is sufficiently warm to keep the overall system above the light-off temperature. Another disadvantage is that, as a result of the considerable richness required, crude emissions are significantly increased. Until ignition of the exhaust gas takes place, crude exhaust gas reaches the environment without any conversion.
A suggested remedy has been to arrange an HC-adsorber instead of the first catalyst in front of the combustion chamber. As a result of this arrangement, HC constituents are intermediately stored, but the second carrier disadvantageously cools down after the heating phase, and loses its activity for a certain interval, until the inflowing exhaust gas has reheated the system to the operating temperature. Furthermore, in the desorption phase, HC reaches the environment from the adsorber without any oxidation in the second monolith.
If the heating phase is extended in order to prevent cooling of the catalyst or catalysts behind the combustion chamber, there is a risk that the catalyst or catalysts may be damaged.
German Patent Document DE 198 04 429 A1 describes a method for rapidly reaching the activation temperature of a catalyst of an applied-ignition internal-combustion engine; this catalyst is arranged close to the engine.
Additional prior art includes International Patent Documents WO 97/25525, WO 93/07365, and WO 92/22734, European Patent Document EP 0 697 242, U.S. Pat. Nos. 5,609,021, 5,685,144, 5,613,360, 5,410,872, and 5,425,233, and British Patent Document GB 2 278 299 A.
It is an object of the present invention to provide an improved method and an improved system for catalytic aftertreatment of internal-combustion engine exhaust gas which permit faster and more efficient heating of the catalyst system.
According to the invention, catalytic aftertreatment of internal-combustion engine exhaust gas is performed by first and second spatially separate catalyst devices provided in an exhaust gas system, with the first catalyst device arranged in a position close to the engine, and the second catalyst device arranged in a position away from the engine. A heating measure, which has an exothermal reaction for heating the second catalyst device, is caused to take place. This reaction may take place in a reaction chamber arranged in front of the second catalyst device.
The invention permits heating of the catalytic emission control system of an internal-combustion engine. The system has a main catalyst arranged close to the engine and a starting catalyst arranged away from the engine. Air is fed to the exhaust gas and is reacted in front of the catalyst away from the engine by applied ignition. Heating takes place during the internal-combustion engine warm-up phase because, during this time period, the catalysts do not have a temperature sufficient for oxidation, for example, of hydrocarbons and carbon monoxide and for reduction, for example, of nitrogen oxides. The special characteristic of the system is the separation of the function of rapidly heating a starting catalyst away from the engine and from the function of a main catalyst which, as a result of its operation close to the engine, reaches the light-off temperature early. This separation permits a shortening of the heating time so that damage to the starting catalyst is avoided as a result of this heating measure.
Advantageous further developments and improvements of the invention are reflected in the claims.
According to one preferred feature of the invention, the heating procedure has a first phase with an exothermal reaction for heating the second catalyst device to a temperature above the light-off temperature.
According to another preferred feature of the invention, the heating procedure has a second phase without the exothermal reaction for cooling to the light-off temperature. In this case, the engine is operated stoichiometrically or in a lean manner. As a result of shortening the heating time of the heating procedure in this way, thermal stressing of the second catalyst device can be considerably reduced.
According to another preferred feature of the invention, the mixture is adjusted such that exhaust gas coming from the engine contains combustible constituents, and air is fed to the exhaust gas with the combustible constituents in the emissions system in order to obtain an ignitable mixture.
According to another preferred feature of the invention, the air is fed at a position upstream of the first catalyst device.
According to another preferred feature, the air is fed at a position downstream of the first catalyst device. This has the advantage that the first catalyst device is better protected against unintentional combustion.
According to another preferred feature, at least two bends are provided in the exhaust gas flow between the position at which the air is fed and the position at which the exothermal reaction takes place. This ensures good mixing of the combustible constituents with the secondary air.
According to another preferred feature, a reaction chamber is set up in front of the second catalyst device. The exothermal reaction is initiated in the reaction chamber by way of an ignition device, preferably a glow device. This ignition device may, for example, be a glow plug or a special spark plug. A catalytically coated glow plug is particularly advantageous, so that the ignition temperature and the ignition energy can be lowered.
According to another preferred feature, the ignition device is essentially arranged centrally in the reaction chamber.
According to another preferred feature, the second catalyst device is provided with a multi-constituent integral coating. The integral coating has a catalytic constituent and an adsorbing constituent for hydrocarbons. The temperature course in the second catalyst device is such that the temperature window between the desorption start and the start of the catalytic conversion is closed. In this HC-adsorber arrangement, on the one hand, the HC-fraction from the exhaust gas can be utilized for exhaust gas afterburning and, on the other hand, the unburnt fraction is intermediately stored in the HC-adsorber which follows and, during desorption, oxidizes simultaneously on the integral coating. The catalyst close to the engine heats up rapidly and reaches its light-up temperature before the integral coating has cooled down.
According to another preferred feature, a third catalyst device is provided upstream directly in front of the reaction chamber of the second catalyst device.
According to yet another preferred feature, the third catalyst device is provided with a multi-constituent integral coating which has a catalytic constituent and an adsorbing constituent for hydrocarbons.
According to still another preferred feature, the reaction chamber is constructed in a cylindrical shape. As a result, an optimal flame front construction can be achieved.
Embodiments of the invention are illustrated in the drawings and are explained in detail below.